The present invention relates to a process for producing hard rolls for use in various industries, for example, for papermaking and fibers, and more particularly to a process for producing hard rolls for use as elastic rolls including papermaking calender rolls, papermaking press rolls, fiber calender rolls, calender rolls for magnetic recording materials, etc.
Generally, paper, fiber, magnetic recording material or like thin material is calendered by being passed between a mirror-surfaced metal roll and an elastic roll opposed thereto at a predetermined temperature and high pressure and made smooth and glossy over the surface by the application of pressure.
Accordingly, the following characteristics are required of the hard roll for use as the elastic roll for calendering (hereinafter referred to as the "calender roll").
(1) Satisfactory surface smoothness. PA0 (2) Hardness, especially surface hardness, remaining unchanged at the operating temperature. PA0 (3) Heat resistance to render the roll less likely to deform due to thermal expansion or melting that would result from autogenous heat. PA0 (4) Compression strength to withstand the high nip pressure applied by the metal roll for the roll to remain free of cracking or breaking.
Conventionally, calender rolls are produced by winding an epoxy resin-impregnated fiber material around a metal roll core to form a lower winding layer covering the core, placing the roll core covered with the winding layer into a mold of predetermined size, and injecting an epoxy resin material into the mold directly to form an epoxy resin covering layer within the mold on curing (see, for example, Examined Japanese Patent Publication SHO 61-15807).
However, epoxy resin or like thermosetting resin generally undergoes marked reaction contraction and thermal shrinkage when curing, so that the contraction or shrinkage is likely to produce a crack in the surface of the hard roll to be produced.
Especially in the case of papermaking calender rolls and the like which are great in diameter and length, the use of epoxy resin or like thermosetting resin for the covering layer entails the problem that a crack develops in the roll surface owing to shrinkage during curing to make it impossible to obtain the roll.